INTRODUCTION 19 



stages of development and also throughout life. Furthermore, the organ- 

 forming, germinal areas of His, and the cells which compose them, were ob- 

 served to be specific in their form and in the possession of organ-specific sub- 

 stances, and His traced this specificity back to the germ layers, to the blastula, 

 and, in the end, to the different parts of the unsegmented egg. It is this 

 specificity to which he attributed the differences in the embryonal develop- 

 ment of different species. Accordingly, Rabl and, in particular, Conklin were 

 able to follow the development of organs from the egg through the first seg- 

 mentations and through later stages to the complete organism ; protoplasmic 

 movements and the character of mitoses were found to correspond to the 

 specific structure of the species. Rabl concluded that the specific characteristics 

 of the organism, or rather of the species, as a whole, determine the specific 

 features of all the organism's component parts — its organs, tissues and in- 

 dividual cells. In the discussion of this investigator we find, therefore, already 

 a suggestion that besides the differences in the organs and tissues which dis- 

 tinguish different parts of the individuals as a species, and even different parts 

 in the unsegmented ova, there is something in the species as such which 

 determines its characteristic development in both the structural and chemical 

 aspects. This species peculiarity became manifest also in transplantations; 

 homoiotransplantations as a rule succeed, while heterotransplantations are 

 unsuccessful. It is evident also in blood transfusions, which may be considered 

 as modified transplantations. 



Oscar Hertwig named the factor which made homoiotransplantation possi- 

 ble, but which caused incompatibility of heterogenous parts of organisms, 

 "vegetative" affinity, and contrasted it with "sexual" affinity which was re- 

 sponsible for successful fertilization. He believed that in plants as well as in 

 animals vegetative and sexual affinity are similar in their manifestation and 

 are due to the same underlying factors. These suggestions of Hertwig were 

 taken up later by W. Schultz, when he tried to prove the parallelism between 

 hybridizability and transplantability in the tissues of vertebrates. However, 

 as we shall see later, while a certain parallelism is noticeable between these two 

 processes, there are also some marked differences. 



It is noteworthy that the concepts of Naegeli and Hertwig related mainly 

 to species, not to individuals. In the meantime, towards the end of the last 

 century, the development of the new science of immunology had set in, but it 

 was likewise primarily concerned with species differences, and only sec- 

 ondarily and somewhat later, with differences between individuals. But the 

 investigators in the field of transplantation and immunity influenced, also, 

 some biologists, as is noticeable in the writing of Fick, who in 1907 added to 

 the concept of the species plasma that of an individual plasma. The fertilized 

 egg of one individual was assumed to differ from those of all other individuals 

 of a certain species in regard to the character of its organ-forming substances ; 

 however, a distinction was made between the living protoplasm, in which 

 these specificities applied, and the trophoplasm, which represents merely food 

 and structural material, and which was less specific or nonspecific. It was 

 recognized that the living protoplasm consists essentially of protein. Each 



